A number of different IMDs comprise hermetically sealed implantable pulse generators (IPGs) or monitors coupled with implantable leads having electrodes for delivering electrical pulses or shocks to and/or sensing electrical activity of a body organ, muscle or nerve and/or bearing sensors, e.g., blood gas, pH, temperature, and pressure sensors. Certain implantable pacing systems and implantable cardioverter/defibrillators (ICDs) employ pressure sensors to monitor cardiac blood pressure through a capacitive pressure sensor in the distal portion of a lead inserted into a patient's heart. Pacing leads having a pressure sensor are used in such cardiac pacing systems to adjust pacing rate in a rate responsive pacing mode, e.g., the VVIR, DDDR, and DDIR pacing modes. Pressure sensors may likewise be utilized on pacing leads for other purposes, such as determining capture by a delivered pace pulse. The pressure sensor on the pacing lead detects the pressure inside the heart chamber into which the lead is inserted, and converts the detected pressure into electronic information which is transmitted back to the pacemaker. The pacing system then analyzes the information received from the pressure sensor and determines an appropriate rate response or uses the information for another purpose. Pressure sensor bearing leads of this type are disclosed, for example, in commonly assigned U.S. Pat. Nos. 5,535,752 and 5,564,434. Pacing systems utilizing ventricular blood pressure, among other physiologic signals, for determining an appropriate rate response are well known in the art, as shown, for example, in U.S. Pat. No. 5,891,175.
In addition, the CHRONICLE.TM. Implantable Hemodynamic Monitor (IHM) made by Medtronic, Inc., assignee of this patent application, continuously monitors right ventricular pressure through such a capacitive pressure sensor bearing lead as disclosed in the '752 and '434 patents inserted into a patient's heart for monitoring the cardiac pressure and EGM and storing such data. A number of other sensors have been proposed to monitor body parameters, e.g., blood gas, pH, temperature and the like for obtaining and using or storing particular information. Such sensors require calibration against standards to ensure that they will accurately measure the body parameter of interest.
For example, it is critical to establish the accuracy of the pressure sensor prior to implanting a pacing or sensing lead having a pressure sensor into a patient. The accuracy of the pressure sensor is not guaranteed, or at least should not be relied upon by physicians, simply based upon the manufacturer's packaging and prior testing of the pressure transducer's accuracy. Once implanted into a patient, the pressure sensor accuracy cannot be easily verified.
The accuracy of the pressure sensor can be divided into two parameters, "baseline pressure" and "scale factor." In regard to the former parameter, the pressure detected by the sensor must accurately reflect the true pressure inside the particular chamber into which it is inserted. Thus, the "baseline pressure" value of the sensor must be established relative to a known applied pressure. In regard to the latter parameter, any increase or decrease in pressure reported by the sensor in the chamber of the heart into which it has been inserted must accurately reflect the amount of pressure actually increased or decreased. Thus, an accurate "scale factor" for the sensor must be established, in order to have complete calibration. In addition, it is preferable that such parameters be adjusted within a sterile environment.
Commonly assigned U.S. Pat. No. 5,919,221 discloses a method and apparatus for testing and calibrating a pressure transducer of the type described in the above-referenced '752 and '434 patents. A calibration vessel is provided having a housing forming a vessel reservoir which, in use, contains the distal portion of the pacing lead having the pressure sensor. The proximal end of the calibration vessel through which the pacing lead is inserted comprises an air tight seal between the vessel housing and the pacing lead. The distal end of the calibration vessel comprises a connector which connects the calibration vessel to either the atmosphere or an input pressure source which functions in connection with a manometer to apply an atmospheric or higher than atmospheric test pressure to the pressure sensor. Test pressure output signals of the pressure sensor under test at atmospheric pressure and under higher than or lower than atmospheric pressure are obtained. The calibration system further comprises an electronic read and display module for reading and displaying the pressure sensor output signal under atmospheric or higher than atmospheric pressure. The reference pressure output signal of an external reference pressure transducer or manometer is also provided to the electronic read and display module for comparison and calibration purposes.
When calibration shows differences between the test and reference pressure signals at sets of test pressures, the differences are encoded as one or more correction factor to be applied to the pressure signal generated by the lead borne pressure sensor under test when it is coupled with an amplifier within the IPG or implantable monitor that the lead is intended to be implanted with. The correction factors are then programmed into memory of the IPG or IHM using an external programmer and the downlink telemetry capability of the IPG or implantable monitor. In one embodiment, the lead pressure sensor is tested while attached to the electronic read and display module to determine the correction factors. In another embodiment, the terminals of the lead under test are attached to the IPG or IHM intended to be implanted in the patient. The interrogation and uplink telemetry capability of the IPG or IHM is employed with an external programmer for commanding that pressure readings at atmospheric and higher than atmospheric test pressures be taken and uplink telemetered and for receiving and displaying the uplink telemetered pressure signals. The uplink telemetered pressure readings are compared with reference pressure readings, and the correction factors are determined and downlink telemetered to the IPG or IHM.
In these tests, it is necessary to break the sterile packaging of the lead and the IPG or IHM to make the necessary electrical connections for the above described testing and calibration. Moreover, it is necessary to provide a particular pressure calibration vessel surrounding the distal portion of the lead including the pressure sensor.
Other functional tests of the IPG or implantable monitor and leads are also conducted prior to or during implantation of various implantable medical devices. For example, stimulation pulse output energies, lead conductor and electrode integrity, sensing thresholds, capture tests and the like are often conducted. A pacing system analyzer is described in U.S. Pat. No. 4,705,042 wherein the pacemaker IPG to be implanted is retained in its sterile package and coupled, along with the implanted pacing lead to the analyzer system. Sterile package connector systems for pacemaker IPGs enabling connection with a pacing system analyzer is also shown in U.S. Pat. Nos. 4,423,732 and 4,605,007.
It would be desirable to conduct tests of the types described above of IMDs employing separately sterile packaged leads and IPGs or implantable monitors adapted to be coupled together when implanted, while they remain in their sterile packages.